It is known that exposure of human or animal tissue to ionising radiation will damage the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, potentially resulting in unacceptable side-effects. It is therefore desirable to design a device for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures over time from a single movable source. The dose deposited from each direction is therefore less than would be required to destroy the tumour, but where the radiation beams from the multiple directions converge, the total dose of radiation is sufficient to be therapeutic. By providing radiation from multiple directions, the damage caused to surrounding healthy cells can be reduced.
Intensity modulated arc therapy (IMAT) is one method of achieving this, and is described in U.S. Pat. No. 5,818,902. In this process, the radiation source is rotated around the patient, and the radiation beam collimated to take a desired shape depending on the angle of rotation of the source, usually with a multi-leaf collimator (MLC). The potential advantages of a particular form of IMAT, volumetric modulated arc therapy (VMAT), have recently given rise to a number of commercial implementations and research studies. In these systems, the dose rate, rotation speed and MLC leaf positions may all vary during delivery. In general, plans comparable in quality and accuracy to static-gantry intensity-modulated radiotherapy (IMRT) can be obtained, normally with reduced delivery times.
To make sure the radiation beams are correctly directed, the treatment can be guided by imaging of the target region, before or even during the treatment. For example, kilovoltage computational tomography (CT) can be used during treatment by providing a separate source of imaging radiation mounted on the rotatable gantry, placed at an angle relative to the main radiation head. A detector is positioned diametrically opposite the source of imaging radiation, and collects imaging data for a plurality of rotational angles of the gantry. This data can then be reconstructed to form three-dimensional images using known CT techniques. See PCT application WO 2006/030181 for an example of this method. Kilovoltage radiation is preferred for imaging due to the high contrast between different structures in the patient.
An alternative method of imaging is to use the megavoltage radiation and an electronic imaging device. In this scheme, a radiation detector is placed on the rotatable gantry diametrically opposite the main treatment head, and is designed to detect the megavoltage radiation after it has passed through (and been attenuated by) the patient. The images generated are therefore individual transmission images, from the beam's eye view (BEV). Megavoltage imaging can be used to verify the position of the MLC leaves in relation to the target within the patient. The aperture thus created by the MLC leaves is known as a portal and hence this form of imaging is often called ‘Portal imaging’ and the detector an ‘electronic portal imaging device’ or EPID. However, the high energy associated with therapeutic radiation is not ideal for imaging purposes as the attenuation coefficients of the various tissue types within a patient are similar at this energy level, leading to poor image contrast. In addition, this method is inherently two-dimensional because in conventional radiotherapy the megavoltage beams are directed at the patient from typically two to nine angles, which may be insufficient to provide three-dimensional imaging.
It has been shown that CT reconstruction from megavoltage images (i.e. MVCT) is possible (see Pouliot J “Megavoltage imaging, megavoltage cone beam CT and dose-guided radiation therapy” 2007 Frontiers of Radiation Therapy and Oncology vol. 40, pp 132-42). However, for such reconstructions, the megavoltage images need to be obtained before or after the delivery of a treatment beam, using beams which generally encompass the anatomy that is desired to be imaged and are therefore not part of the radiation treatment. As this method does not make use of portal images acquired during treatment (i.e. those acquired with the varying MLC aperture of therapy) it is associated with an increase in undesired radiation dose to the patient.
A paper by Ruchala et al (“Megavoltage CT imaging as a by-product of multileaf collimator leakage”, 2000 Physics in Medicine and Biology, vol. 45, pp N61-70) discloses a method of reconstructing three-dimensional CT images in tomotherapy. This process utilizes the leakage radiation through the closed leaves of a binary multi-leaf collimator (MLC), along with slight inefficiencies in treatment delivery, to generate MVCT images during treatment. However, the process is applicable only to tomotherapy, in which the leaves of the MLC are either open or closed, i.e. binary. The portal images for CT reconstruction are acquired only when all leaves of the MLC are in their closed positions, i.e. the leakage radiation used to create the images also generally encompasses the entire anatomy that is desired to be imaged.
What is required is an apparatus and a method for providing images of a target region in a patient during radiotherapy. Conventional kilovoltage CT scanning requires significant additional equipment (e.g. an extra source of radiation and a detector), leading to increased complexity and cost. Two-dimensional portal imaging suffers from reduced contrast between different internal structures, and it is frequently necessary to supplement it by larger and/or orthogonal images taken prior to or after treatment. Megavoltage CT and these other two approaches therefore increase the undesired dose applied to the patient. These techniques also potentially increase the time required to treat the patient as they represent an additional task for the operator to perform.